The preparation and characterization of the µ-amido-µ-oxalato-bis[tetra-amminecobalt(III)] complex, and the kinetics of reduction with Cr2+ and V2+

Abstract

The chloride salt of the protonated form of the µ-amido-µ-oxalato-bis[tetra-amminecobalt(III)] complex, has been prepared and characterized. The perchlorate salt has also been prepared and it would appear that the oxalate [graphic omitted] bridge is only 50% protonated in this solid. From spectrophotometric measurements on the complex in acidic solutions at 15–45°, µ= 2·0M(LiClO4), values of the protonation constant K= 18 ± 2 l mol–1 have been determined. Reduction with chromium(II), 2Cr2++(CoIII)2→ 2Co2++(CrIII)2, proceeds in two stages, each involving attack of the chromium(II) and reduction of cobalt(III). Both protonated (ka) and unprotonated (kb) forms participate in the first stage of the reaction (k1), and the rate law (Cr2+ in large excess) is as in (i) [graphic omitted]. At 25° and µ= 2·0M(LiClO4), ka= 1·45 ± 0·07 l mol–1 s–1, and kb= 35·2 ± 0·4 l mol–1 s–1. The second stage of the reduction is independent of [H+] as in (ii) [graphic omitted] and k2= 1·88 ± 0·05 l mol–1 s–1 at 25°. Information regarding the spectrum of the intermediate and final product has been obtained. The reduction with V2+ also proceeds in two stages, the first of which is directly measurable. The rate law observed is of the same form as (i), and again both the protonated (kc) and unprotonated (kd) forms are reactive. At 25° and µ= 2·0M(LiClO4), kc= 3·51 ± 0·04 l mol–1 s–1 and kd= 22·7 ± 0·4 l mol–1 s–1. Activation parameters have been obtained for ka, kb, k2, kc, and kd.